<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }

        body {
            width: 24rem;
            height: 13.5rem;
            border: 3px solid red;
            box-sizing: border-box;
        }

        ul {
            display: flex;
            flex-direction: row;
            flex-wrap: wrap;
            width: 100%;
            height: 100%;
        }

        li {
            width: 33.333%;
            height: 50%;
            font-size: 0.375rem;
            list-style: none;
            border: 3px solid green;
            box-sizing: border-box;
        }
    </style>
</head>

<body>
    <ul>
        <li>1</li>
        <li>2</li>
        <li>3</li>
        <li>4</li>
        <li>5</li>
        <li>6</li>
    </ul>
</body>
<script src="./lib-flexible.js"></script>

</html>